[BOLT][test] Enable GNU extensions where necessary

With a Clang that doesn't default to GNU extensions they need to be enabled explicitly.

Author: tambry

bolt/test/AArch64/data-at-0-offset.c

@@ -1,4 +1,4 @@
-// RUN: %clang %cflags -O2 -fPIE -Wl,-q -pie  %s -o %t.exe
+// RUN: %clang %cflags -O2 -fPIE -std=gnu99 -Wl,-q -pie  %s -o %t.exe
 // RUN: llvm-bolt %t.exe -o %t.bolt 2>&1 | FileCheck %s
 // CHECK-NOT: BOLT-WARNING: unable to disassemble instruction at offset
 

bolt/test/runtime/X86/instrumentation-indirect.c

@@ -38,7 +38,7 @@ int main(int argc, char **argv) {
 /*
 REQUIRES: system-linux,bolt-runtime,lit-max-individual-test-time
 
-RUN: %clang %cflags %s -o %t.exe -Wl,-q -pie -fpie
+RUN: %clang %cflags -D_GNU_SOURCE %s -o %t.exe -Wl,-q -pie -fpie
 
 RUN: llvm-bolt %t.exe --instrument --instrumentation-file=%t.fdata \
 RUN:   --instrumentation-wait-forks=1 --conservative-instrumentation \